This invention relates generally to operational amplifiers and, more particularly, to CMOS rail-to-rail operational amplifiers.
Operational amplifiers are well known electrical components that typically amplify a small differential input voltage to provide a highly amplified single-ended output voltage. CMOS amplifiers are typically powered by a positive power supply voltage designated V.sub.DD and a negative power supply voltage designated V.sub.ss, sometimes referred to as "power supply rails" or simply "rails". One limitation of operational amplifiers, including CMOS operational amplifiers, is that the input section cannot sense voltage throughout the entire voltage range determined by the power supply voltages. This is due to the constraints of most circuit designs, wherein a volt or more is necessary to bias the transistors in the input section. Consequently, when the input voltage is within a volt of the power supply rails it will not be sensed by the input section. Similarly, the output voltage typically cannot track through the entire voltage range because of the necessary bias voltage overhead in the output section.
Referring now to FIG. 1, a maximum voltage, V.sub.MAX, and a minimum voltage, V.sub.MIN, determine the effective operational range for the operational amplifier. The voltages V.sub.MAX and V.sub.MIN can be different for the input and output sections and are determined not only by circuit design, but by semiconductor process limitations and environmental factors such as temperature.
It is desirable to increase the operational range for both the input and output voltages as much as possible, and even up to the supply voltages, i.e., full "rail-to-rail" operation. While full rail-to-rail operation is desirable, existing CMOS designs are extremely complex and include many active devices. The complex designs increase chip area and cost. An example of a prior art CMOS rail-to-rail operational amplifier is shown in "A Rail-to-Rail CMOS Op Amp" by J. N. Babanazhad, IEEE Journal of Solid-State CircuitsVol. 23. No. 6, December 1988, pp. 1414-1417.
Accordingly, a need remains for a CMOS operational amplifier that has full rail-to-rail operation while having a simple circuit design using a minimum number of transistors.